1. Field of the Invention
The present invention relates to a phosphor and a light emitting diode using the same, and more particularly, a light emitting diode composed of a phosphor which has an excitation and emission characteristic for absorbing light emitted from a light emitting diode (LED) chip and emitting light suitable to realize white light.
2. Discussion of the Background
In general, a white light emitting diode is a light emitting device which is PN junction diode composed of a compound semiconductor and receives an voltage to emit light, so that a single chip or multiple chips may be used to realize white light.
When multiple chips are used to realize white light, the respective light emitting diodes are used as red, green and blue light emitting sources, and thus, the color rendering is very good. However, there are some problems in that it is not easy to mix colors due to the brightness difference of red, green and blue colors, the operating voltages applied to respective chips are uneven, and the outputs of respective chips are changed depending on the ambient temperature to cause the color coordinate to be changed.
On the other hand, when a single chip is used, a LED chip composed of compound semiconductor and a phosphor are combined to realize white light. To this end, on a blue light emitting LED chip, a phosphor using a portion of the blue light as an excitation source to emit yellow-green or yellow light is attached such that the blue light emitted from the LED chip and the yellow-green or yellow light emitted from the phosphor are mixed into the white light. Currently, an yttrium-aluminum-garnet based phosphor disclosed in Japanese Patent No. 2,927,279 or a combination of green and red phosphors for use in color televisions has been practically used as a phosphor.
That is, a white light emitting diode is obtained from a combination of a blue LED chip composed of a GaN semiconductor component within a wavelength range of 430 nm to 480 nm and a phosphor capable of emitting yellow light using the blue light as an excitation source. In general, this white light emitting diode so configured is widely employed since it is inexpensive and very simple in view of its principle and structure.
An yttrium-aluminum-garnet (YAG:Ce) based phosphor having typically excellent excitation capability by blue light, an orthosilicate phosphor having a chemical formula represented by Sr2SiO4:Eu and a thiogallate phosphor represented by CaGa2S4:Eu have been generally used as a phosphor for use in the white light emitting diode.
However, in order to synthesize the yttrium-aluminum-garnet based phosphor and the orthosilicate phosphor, there are problems in that high-temperature heat treatment conditions, very sophisticated purity control and exact stoichiometry are required. Further, in order to use the light emitting diode as a general lamp, a phosphor with higher luminous efficiency and intensity is required. Currently, studies for solving the aforementioned problems to develop a more economic synthetic method and for developing a phosphor with which the yttrium-aluminum-garnet phosphor and the orthosilicate phosphor can be replaced have been actively conducted throughout the world.
The thiogallate phosphor used to overcome the aforementioned problems is expressed as a general formula AB2S4:Eu, wherein A is at least one element selected from the group consisting of Ca, Sr and Ba, and B is at least one element selected from the group consisting of Al, Ga and In. More specifically, the SrGa2S4:Eu and CaGa2S4:Eu phosphors emit relatively intense green and yellow light, respectively.
The thiogallate phosphor has the same advantage as the yttrium-aluminum-garnet phosphor and the orthosilicate phosphor in that a variety of light emitting colors can be implemented by controlling the kinds and concentrations of elements in a base material and luminescence center (activator).
U.S. Pat. No. 3,639,254 as the prior art related to the thiogallate phosphor discloses a thiogallate phosphor and a manufacturing method thereof in which Eu, Pb and Ce are used as luminescence center (activator). In this patent, a number of embodiments with various composition providing a wide range of light emitting colors depending on the kinds and concentrations of the luminescence center (activator) are described. Further, U.S. Pat. No. 5,834,053 discloses a blue light emitting thiogallate phosphor with a crystalline micro-structure and a method for manufacturing the phosphor using a chemical vapor deposition (CVD). Specifically, the method for manufacturing an electroluminescent device with a thin film layered structure by providing a low-temperature process, in which the conventional high-temperature annealing step is eliminated, such that a general glass panel can be used on behalf of a special panel made of a glass-ceramic material.
The above U.S. Pat. Nos. 3,639,254 and 5,834,053 disclose the thiogallate phosphors whose emission spectra exist within a blue or green spectrum range. These phosphors are expressed as AGa2S4, wherein A is at least one element selected from the group consisting of alkali earth metal specifically including Ca, Ba, Sr and Zn. The luminescence center (activator) is Eu, Pb or Ce.
However, the luminous efficiency of the above phosphor is very low as compared with application fields (e.g., the illumination engineering) in which light with higher luminous efficiency is required.
As another prior art, U.S. Pat. No. 6,695,982 discloses a thiogallate phosphor with high luminous efficiency and a manufacturing method thereof in order to solve the problems related to the low luminous efficiency. In this patent, the ratio of the divalent ions A to the trivalent ions B in the thiogallate phosphor with the general formula AB2S4 is selected to differ from the ratio A:B=1:2. Therefore, higher luminous efficiency can be obtained and different light emitting wavelength and color positions can be accomplished.
However, U.S. Pat. No. 6,695,982 merely discloses that a position A is replaced with an ion such as Zn or Mg ion with an ion size significantly smaller than Ca ion whereas a position B is replaced with another ion such as Y ion with an ion size significantly larger than Ga ion only in consideration of charge balance. This inconsistency of ion sizes may cause the crystal to be twisted and distorted, and thus, the luminescent characteristic will be greatly deteriorated.